1) Field of the Invention
This invention relates to an optical modulator suitable for use in the field of long distance optical communication systems.
2) Description of the Related Art
As data transmission rates have increased in recent years, optical modulators for modulating a data signal from an electric signal into an optical signal are being developed energetically in the field of long distance communication systems such as submarine optical communication.
An example of an optical modulators as just described is dual drive optical modulator 30 as shown in FIG. 22. Referring to FIG. 22, the dual drive optical modulator 30 shown includes a substrate 31 on which a Mach-Zehnder optical waveguide 32 is formed, and an electrode 33 formed integrally on the substrate 31 and including two signal electrodes 33A-1 and 33A-2 and a grounding electrode 33B. The dual drive optical modulator 30 modulates incoming light from a light source not shown with an NRZ data signal.
FIG. 23 is a sectional view taken along line A—A′ of the dual drive optical modulator 30 shown in FIG. 22. As seen in FIG. 23, the dual drive optical modulator 30 is configured such that the electrode 33 is integrally formed on the substrate 31, which is made of, for example, lithium niobate (LiNbO3) and cut in the Z-axis direction of the crystal orientation (Z-axis cut), together with the Mach-Zehnder optical waveguide 32.
The Mach-Zehnder optical waveguide 32 is formed by thermal diffusion of titanium (Ti) or a like substance on the substrate 31 and includes a Y branching waveguide 32A and two straight arm waveguides 32B-1 and 32B-2 on the light incoming side and a Y branching waveguide 32C on the light outgoing side.
The electrode 33 is formed partially on the substrate 31 with a buffer layer 35 (refer to FIG. 23) interposed therebetween and includes the two signal electrodes 33A-1 and 33A-2 and the grounding electrode 33B.
The electrode 33 can modulate incoming light into an NRZ optical signal by applying NRZ data signals from NRZ data signal generators 34A and 34B which are hereinafter described as electric signals to the signal electrodes 33A-1 and 33A-2.
As shown in FIG. 22, the signal electrodes 33A-1 and 33A-2 are each formed so as to establish an electric connection between two connector contacts on a one-side edge portion of the substrate 31 in its widthwise direction. Further, the signal electrode 33A-1 is formed such that part of it extends along and above the portion at which the straight arm waveguide 32B-1 is formed. Further, the grounding electrode 33B is formed such that it is disposed on the opposite sides of the signal electrodes. 33A-1 and 33A-2 in a spaced relationship by a predetermined distance thereby to form a coplanar line on the substrate 31.
The NRZ data signal generator 34A applies a voltage signal (microwave) as an NRZ data signal to the signal electrodes 33A (33A-1 and 33A-2). The NRZ data signal generator 34B applies a voltage signal (microwave) as an NRZ data signal to the signal electrode 33B.
When light from a light source (not shown) is introduced into the dual drive optical modulator 30 having the configuration described above with reference to FIG. 22, while the light propagates in the Mach-Zehnder optical waveguide 32, it is modulated into an NRZ optical signal by the signal electrodes 33A-1 and 33A-2 to which a voltage signal (microwave) of NRZ data or the like is applied.
In order to design an optical device for which high speed operation is required such as an optical modulator described above, it is necessary as a basic design item to take several parameters into consideration including (1) the drive voltage, (2) the velocity match between the optical signal and the electric signal, (3) the attenuation constant of the electric signal, (4) the characteristic impedance (normally 50Ω) (5) the wavelength chirp amount, and (6) the loss. Particularly, it is important for improvement in power consumption and transmission quality of the apparatus to lower the drive voltage of the optical device.
Where such a dual drive optical modulator as described above is used to modulate a voltage signal into a data optical signal of a transmission rate particularly of 10 Gb/s or more, preferably of approximately 40 Gb/s, it is a significant subject for improvement of the transmission quality to lower the drive voltage while arbitrating with the values of the other evaluation parameters such as the velocity match between the optical signal and the electric signal as described above.